Preparation of silver particles using thermomorphic polymers

ABSTRACT

This invention relates to the synthesis and isolation of colloidal silver particles through the use of thermomorphic polymers and the resulting composition. It further relates to the use of the resulting composition in the preparation of inks for printing with silver-containing inks.

FIELD OF THE INVENTION

This invention relates to processes for the synthesis and isolation ofcolloidal silver particles through the use of thermomorphic polymers,and compositions containing the colloidal silver particles. Theinvention further relates to the preparation and use of inks forprinting of silver, from the colloidal silver particles preparedaccording to the processes disclosed herein.

BACKGROUND

Continuing miniaturization in the electronic industry is driving thefeatures size of printed conductors smaller. To be able to print smallerconductors, the particle size of the conductive particles in inks havebecome smaller. In the manufacture of silver particles or colloids on acommercial scale, isolation of the dry product is an issue. There arereasonable techniques that work well for particles down to a micron, butbelow that isolation is difficult. A process that eases isolation wouldbe of commercial value.

Processes known for the isolation of silver nanoparticles includefiltration, centrifugation and spray drying. Filtration can becomeproblematic or slow when the particle size is very small. Centrifugationcompacts the particles into a single mass, often defeating the purposeof having made small particles. Spray drying can be an effective methodof isolation if the particles will hold together resulting in no fines,but it provides no method of removing ionic byproducts.

SUMMARY OF THE INVENTION

The present invention provides processes for the preparation andisolation of colloidal silver particles by carrying out the reduction ofa silver salt in the presence of an amine base and a thermomorphicpolymer, wherein the thermomorphic polymer passes through its transitiontemperature carrying the colloidal silver articles out of suspension,thereby allowing facile isolation and washing of the product.

One aspect of the present invention is a process for preparing andisolating silver particles or colloids comprising:

-   -   a) providing, at a first temperature, a combined mixture        comprising 1) a first aqueous solution of a silver(I) salt and        an amine and 2) a second aqueous solution comprising a reducing        agent; said combined mixture further comprising a thermomorphic        polymer having a transition temperature, said thermomorphic        polymer being in a homogenous phase at said first temperature;        and    -   b) changing the temperature of the combined solution from the        first temperature to a second temperature at which the        thermomorphic polymer is in a heterogeneous phase, such that the        thermomorphic polymer and silver separate from the combined        solution to form an agglomerate; and    -   c) isolating the agglomerate from the reaction medium.

In some embodiments, the silver salt is selected from silver nitrate,silver trifluoroacetate, silver oxide and silver acetate.

In some embodiments, the silver(I) salt, the amine and the thermomorphicpolymer are provided in the first solution, and the reducing agent isprovided in the second solution, and the combined mixture is formed bycontacting the first solution and the second solution.

In some embodiments, the silver(I) salt and the amine are provided inthe first solution, and the thermomorphic polymer and the reducing agentare provided in the second solution, and the combined mixture is formedby contacting the first solution and the second solution.

In further embodiments, the silver(I) salt and the amine are provided inthe first solution, the reducing agent is provided in the secondsolution; the first solution and the second solution are combined toform an admixture; and the admixture is contacted with the thermomorphicpolymer to form the combined solution prior to changing the temperatureof the combined solution from the first temperature to a secondtemperature at which the thermomorphic polymer is in a heterogeneousphase, such that the thermomorphic polymer and silver separate from thecombined solution to form an agglomerate.

Another aspect of the present invention is silver particles prepared andisolated by the process described above.

A further aspect of the invention is a process for the preparation ofsilver-containing inks.

Another aspect of the invention is silver-containing ink formulationsmade with the silver particles produced by the processes, and processesto produce the silver inks.

A further aspect of the invention includes processes for printing usingthe silver-containing inks. Such processes include inkjet, screen,gravure, offset, spin, or contact printing. Printing processes can beused for printing patterns and images, particularly for semiconductorapplications.

These and other aspects of the invention will be apparent to one skilledin the art in view of the following description and the appended claims.

DETAILED DESCRIPTION

Colloidal-, micron-, or submicron-sized silver particles are typicallyprepared on an industrial scale by the reduction of silver salts. Tocontrol the size, size distribution, morphology and other aspects of theresulting particles, a variety of agents are added to the reductions tocontrol the precipitation of the resulting silver metal. Stabilizationand isolation of the resulting silver particles is a critical aspect tothe commercial technologies.

The following series of reactions represent the chemical transformationstaking place during the synthesis of the colloidal silver with analdehyde reducing agent. An aqueous solution of an amine provides abasic environment.4 H₂O+4 RNH₂→4 RNH₃ ⁺OH⁻The base reacts rapidly with the silver ion precipitating silver oxide.2 Ag⁺+2 OH⁻→Ag₂O↓+H₂OThe presence of amine in the solution resolubilizes the silver as thediamine-stabilized silver hydroxide.Ag₂O↑+4 RNH₃ ⁺OH⁻→2 [Ag(RNH₂)₂]⁺OH⁻+3 H₂OThe existing basic conditions facilitate the reduction of silver(I) tosilver(0) by aldehydes or other reducing agents.R′C(O)H+2[Ag(RNH₂)₂]⁺OH⁻→2 Ag↓+R′CO₂ ⁻RNH₃ ⁺+H₂O+3 RNH₂Summing the above equations gives the overall stoichiometry for thereaction.2 Ag⁺+2 OH⁻+RNH₂+RC(O)H→2 Ag↓+H₂O+R′CO₂ ⁻NRH₃ ⁺It should be noted that while there is but one alkylamine in the overallstoichiometry, a stoichiometry of two alkylamines to every silver ion isrequired to keep the silver in solution during the reduction process.The aldehyde reducing agent is used here for illustrative purposes, butany of a variety of reducing agents will work in this reaction.

The above paragraph described the underlying synthetic chemistry of thisprocess, but not the role of additives that can be used to control theshape or morphology, size, size distribution, dispersability and otherproperties of the precipitated product. Those additives control thephysical but not chemical course of the reaction by modifying particlenucleation, rates of growth of particular crystalline faces, andagglomeration of growing particles. An entire edition of the MRSBulletin (May 2005, 30(5) and particularly the figure on page 339)describes the wide range of additives available and their applications.The additives can affect the performance properties of the manycommercially-available products in the market place. The processtechnology described herein is directed at the isolation step and isgenerally independent of those additives. Nonetheless, it can impact theisolation of all of those various particle morphologies.

As used herein, the term “silver(I) salt” refers to any of a variety ofcompounds of silver in the plus one oxidation state. Most commonly,silver nitrate is desirable because it is the least expensive and mostreadily available of the silver(I) compounds. Nonetheless, silver(I)salts also include silver(I) compounds having other counterions such asacetate, carbonate, chlorate, chloride, citrate, cyanate,cyclohexanebutyrate, diethyidithiocarbamate, fluoride,hexafluorophosphate, lactate, methanesulfonate, nitrite, oxide, sulfate,tetrafluoroborate, trifluoroacetate, or trifluoromethanesulfonate.Additionally, “silver(I) salt” includes the above silver(I) salts thatadditionally include a ligand coordinated to the silver center. Examplesof ligands that may be coordinated include amines and diamines such asmethylamine, ethylenediamine, ethanolamine, diethanolamine, anddiethylamine.

As used herein, the term “thermomorphic polymer” refers to a polymerthat has solubility in a solvent at one temperature and undergoes aclean and rapid transition to insolubility at another temperature. Forexample, poly(N-isopropyl acrylamide) displays an inverse solubility inwater in that it is soluble in cold water but precipitates cleanly fromsolution upon going through a critical transition temperature. Thus, thethermomorphic polymer/water system is monophasic when it is cold andbiphasic when it is hot. The transition temperature is dependent upon anumber of features including molecular weight, solvent composition, andpossible comonomer incorporation.

More commonly, polymers are soluble at higher temperatures andprecipitate at lower temperatures. They have phase selective solubilityin thermomorphic solvent mixtures that are biphasic when cold andmonophasic when hot.

As used herein, the terms “heterogeneous phase” and “homogeneous phase”refer to the thermomorphic polymer and its state relative to the aqueoussolvent. “Homogeneous phase” implies that the polymer is largely insolution and the system appears homogeneous to the eye. When in thestate where the polymer is a “heterogeneous phase,” the polymer is notin solution, but rather, has precipitated from solution as a distinctsecond phase obvious to visual inspection. The terms heterogeneous phaseand homogeneous phase are not intended to refer to the state of thesilver, which will be in a homogeneous solution prior to reduction and aheterogeneous phase after reduction.

As used herein, the term “transition temperature” is a temperature orrange of temperatures over which the solubility of the thermomorphicpolymer solution undergoes a phase transition from homogeneous toheterogeneous. Polymers exhibiting either normal or inverse thermalbehavior can be employed in the processes disclosed herein, so long asthe transition is substantially complete and rapid. By substantiallycomplete and rapid is meant that greater than 90 percent of the polymerprecipitates from solution within a temperature range of 5° C., withinthe time required to effect the temperature change. The homogeneous(monophasic) range may be below or above the transition temperaturedepending upon the particular thermomorphic polymer chosen. For a givenwater/polymer system the transition temperature will be dependent uponthe molecular weight of the polymer, the presence of ions or otheradditives in the system, and the presence of other polymers orcosolvents. The transition temperature may be determined by “cloudpoint” measurements well known to those skilled in the art (See forinstance, H. Hoffmann, Tenside Detergents 11(1), 30-1 (1974))

When the transition through the transition temperature of the polymeroccurs and the thermomorphic polymer/solvent system becomesheterogeneous in the presence of an additional heterogeneous colloidalor nanoparticle phase, the polymer phase will carry the colloid fromsolution into the polymer phase. This process of polymer precipitationin the presence of a heterogeneous silver phase, carrying the polymerand entrapped silver out of solution is referred to as “coagulation.”

As used herein, the term “amine” refers to ammonia or a variety of alkylamines known to associate with Ag(I) in solution. These include aminessuch as methylamine, dimethylamine, ethanolamine, diethanolamine,diethylamine and other related species. It also includes diamines orpolyamines such as ethylenediamine, N-methylethylene-diamine,N,N′-dimethylethylenediamine, and di(2-aminoethyl)amine. The purpose ofthe amines is twofold in that they solubilize the silver(I) by ligatingthe silver center under basic conditions of the reaction rather thanallowing the silver to precipitate as AgO; and silver ions under basicconditions when ligated with a pair of amine ligands, [Ag(NH₂R)₂]⁺, aremore easily reduced to silver metal than unligated silver(I).

As used herein, the term “supporting polymer” means a polymer added inaddition to the thermomorphic polymer to control the course of thereaction or particle morphology or size. Such control of morphology iswell known to those experienced in nanoparticle formation (see forinstance, Benjamin Wiley et al.; Chemistry, 11(2), 454-63 (2005)). Thesupporting polymer does not act as a thermomorphic component in thesystem. Supporting polymers are generally commercially availablepolymers. One or more supporting polymers can be used independently ortogether. The polymers may be copolymers, interpolymers or mixturesthereof. The polymers may be made from nonacidic comonomer comprisingC₁-C₂₀ alkyl methacrylate, C₁-C₂₀ alkyl acrylates, styrene, acrylamide,substituted styrene, vinyl acetate, vinyl pyrrolidinone or combinationsthereof. The copolymers and interpolymers may further include acidiccomonomers comprising ethylenically unsaturated carboxylic acidcontaining moieties. The copolymer, interpolymer or mixture thereof hasan acid content of between 0 and 30 weight percent of the total polymerweight. The polymers will generally have an average glass transitiontemperature (Tg) of 50 to 150° C. and weight average molecular weight inthe range of 2,000 to 250,000 and all ranges contained therein.Typically, the supporting polymer can be a poly(acrylamide),poly(ethylene oxide) or copolymer of vinyl acetate andvinyl(pyrrolidinone). Addition of the supporting polymers can affect thesize, crystallinity, morphology and agglomeration of the silverparticles and therefore, their utility in end-use applications. The“supporting polymer” may be a surfactant or it may be impossible todistinguish between a surfactant and a supporting polymer because thereis a continuum of molecular weights and a continuum of surface-activeproperties between the two extremes. Nonetheless, both supportingpolymers and surfactants can be present during the process.

As used herein, the term “surfactant” means a molecule added to any stepin the process to control the course or outcome of the reaction or thematerial properties of the product. Addition of surfactants can have aneffect on the size, crystallinity, morphology and particularly theagglomeration and dispersability or redispersability of the silverparticles and therefore, their utility in end-use applications. Thesurfactant can be used in addition to or in place of any supportingpolymer in the system. The surfactants may also be referred to asdispersants, particularly when they are employed in the formulation ofinks and other products. Such technology is well known to those skilledin the art, and is disclosed, for example, in Catherine J. Murphy etal., Advanced Materials (Weinheim, Germany) 13(18), 1389-1393, (2001)for surfactant effects on nanoparticle growth; U.S. Pat. No. 5,389,122;and PCT Int. Appl. WO 2005037464 (2005) for surfactant effects ondispersion.

The surfactants can be present in the initial production of the silverparticles by reduction, or can be added subsequent to reduction. Ifsurfactants are present in both steps, they need not be the samesurfactant. Thus, for example, the surfactant cetyltrimethylammoniumbromide can be present during the reduction step to control particlemorphology and stearic acid can be present during particle isolation orwashing to maintain the dispersability of the silver particles insubsequent end-uses.

The coated silver particles may be at least partially coated with thesurfactant. The surfactant may be selected from stearic acid, palmiticacid, a salt of stearate, a salt of palmitate and mixtures thereof, andcontain a counter-ion selected from hydrogen, ammonium, sodium,potassium and mixtures thereof. Additionally, the surfactant may includelong-chain alkyl ammonium compounds such as cetyltrimethylammoniumbromide.

If a mixture of stearic and palmitic acids or their salts are used, itis preferred that the weight percent of stearic acid or stearate be from30 to 70 percent and the weight percent of acid in an acid salt mixturebe 30 to 100 percent. The total amount of surfactant is preferably from0.10 to 1 weight percent based on the weight of the silver particles,whether the surfactant is on the silver particles or added to thecomposition.

As used herein, the term “reducing agent” is any substance capable ofbringing about the reduction of silver(I) to precipitated silver(0)through the donation of electrons as it itself is oxidized. It isrecognized that silver ions ligated with a pair of amine ligands,[Ag(NH₂R)₂ are more easily reduced than unligated silver(I). Examples oforganic reducing agents include resorcinol, 4-butyrolactone, furfural,manitol, 1,4-cyclohexanediol, guaicol, I-ascorbic acid, its salts andrelated compounds such as sodium ascorbate, d-isoascorbic acid, etc. andrelated compounds having a ring of the ascorbic acid type such ashydroquinone, quinone, and catechol, reducing sugars, organic hydrazinesand hydroxylamines, dimethylformamide, ethylene glycol and otheralcohols, and aldehydes such as formaldehyde, acetaldehyde orpropionaldehyde. Formaldehyde is particularly convenient, leading tocoproducts that are easily removed from the system. Inorganic reducingagents such as hydrazine, sulfite salts, formate salts, borohydridesalts, TiCl₃, or FeCl₂, could also be employed. Lewis acidic reducingagents such as the Fe or Ti compounds may require modification of theamine concentration and other stoichiometries in the reaction becausethey can remove some amine through Lewis acid/base interactions.

The thermomorphic nature of the polymers can be utilized not only in thesynthesis of the colloidal silver but also in subsequent purification.For example, after synthesis, the coagulated or agglomerated reactionmass can be re-suspended in cool water. This allows redissolution of thepolymer phase and intimate contact of the silver particles with awashing medium. After an appropriate period of time, heating of themixture will cause re-coagulation and the washing medium may be decantedor filtered. This procedure can be repeated multiple times until somepredetermined criterion has been achieved. One such criterion is theconductivity of the wash water, for example, when the conductivity ofthe wash water falls below 10 microohms.

During the synthesis and washing procedures, it may be useful for asurfactant such as stearic acid be present. It is possible that thesurfactant can be washed from the silver particles if additionalsurfactant is not present during the washing procedure. The desirabilityof the presence of the surfactant in the wash water during the washingprocedure is dependent upon the envisioned end-use of the resultingsilver.

When formulating silver-containing inks, the silver materials disclosedherein are dispersed in an “organic medium.” The main purpose of theorganic medium is to serve as a vehicle of the dispersion of the finelydivided solids of the composition in such form that it can readily beapplied to a ceramic, glass or other substrate. Thus, the organic mediumis desirably one in which the solids are dispersible with an adequatedegree of stability. Secondly, it is desirable that the organic mediumhas rheological properties that improve the performance of the resultingdispersion in the printing applications. Organic media for screenprinting of silver inks and more particularly for screen printing ofsilver inks to be photoimaged are described in Glicksman (PCTApplication WO03/032087 to DuPont) which is incorporated by referenceherein in its entirety. Organic media for inkjet printing, particularlyfor inks containing high levels of silver particulate are quitedifferent in nature and are described in copending application U.S. Ser.No. 10/775,785), which is incorporated by reference herein in itsentirety.

In a typical process, an aqueous solution of the thermomorphic polymer,e.g., poly(isopropylacrylamide) and silver nitrate is combined withpoly(acrylamide). Poly(isopropylacrylamide) is preferred because it hasa rather clear transition temperature in water solution and it has theunusual added benefit of being insoluble above the transitiontemperature, as opposed to most other polymers, which are soluble at thehigher temperatures and insoluble at lower temperatures. This inversesolubility behavior is of benefit during the synthesis and particularlyduring subsequent washing steps because the byproducts of the reactionare more soluble at the higher temperatures so their removal from thefinal product is more complete.

The poly(acrylamide) is added to control the morphology of the silverparticles. While the poly(isopropylacrylamide) seems to play a similarrole in morphology in addition to its thermomorphic role, it is notbelieved to be as effective as the unsubstituted poly(acrylamide).Nevertheless, it is not intended that the present invention be bound byany particular theory.

When methylamine is added to the aqueous polymer silver solution, thereis an instantaneous change in pH, causing the momentary formation of abeige precipitate of basic silver oxide. The beige precipitate goes backinto solution as the amine-stabilized silver cationic complex is formed.The addition of amine is preferably carried out rapidly with effectivestirring. Some silver colloid formation during this step is evidenced bythe discoloration of the resulting solution.

At this point, a solution of the reducing agent is added causing rapidformation of silver precipitate. An aqueous solution of formaldehyde(usually containing some residual methanol) is a convenient choice ofreducing agent because the byproducts of the reaction are easily removedfrom the reaction medium. Hydroquinone is another possible choice ofreducing agent, but it can result in the formation of an insolublebyproduct, the crystalline 1:1 charge transfer adduct of hydroquinoneand quinone.

Upon addition of the reducing agent there is an immediate reactionprecipitating the colloidal silver. It is possible to choose an initialreaction temperature such that the exotherm of the reduction will takethe temperature of the reaction mass above the transition temperature ofthe thermomorphic polymer thereby spontaneously precipitating thereaction mass or coagulant. Otherwise, the reaction may be heated tocause the coagulation.

When the reaction is complete, a coagulated or agglomerated product massforms and separates from the solution. When the mass separates from thesolution it generally does so as a viscous mass. The supernatent isgenerally clear, the polymer having carried virtually all of the silverout of solution when it precipitated. The supernatent may be coloredslightly tan but this represents a minimal loss of silver. Thethermomorphic polymer is maintained at a temperature well above itstransition temperature during the isolation processes because otherwise,the reaction coagulant can return to a mobile solution phase makingisolation of the silver problematic. The supernatant is preferablydecanted from the reaction coagulant, taking most of the reactionbyproducts with it. Alternatively, the mass may be filtered.

If it is desired, further washing of the product mass is easilyaccomplished. The product mass may be re-suspended in an excess of freshwater at a temperature below the transition temperature of the polymer.Stirring will result in a mobile slurry of silver with most if not allof the polymer going into solution. The solution is then reheated withstirring until the polymer once again precipitates, taking the silverout of suspension. The supernatent can be decanted once again. Thisprocedure may be repeated as many times as required to achieve the levelof purity desired.

Poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)(PEO-PPO-PEO) triblock copolymers such as the Pluronic®) polyethersavailable from BASF (BASF Corporation, Mount Olive, N.J.) are desirablefor use in the processes, because they have good surfactant abilities,and low toxicity. For example, aqueous solutions of Pluronic® F127copolymers exhibit temperature-induced aggregation phenomena as a resultof the hydrophobic nature of the PPO block. Gelation of concentratedPluronic® F127 solutions occurs upon heating to temperatures at or justabove ambient, making them suitable for the processes described herein.

The described process may also be carried out with thermomorphicpolymers having a more conventional temperature behavior—e.g.,solubility at high temperatures and insolubility at lower temperatures.In general, transitions in such polymers are more gradual than those inthermomorphic polymers, with solubility increasing as a linear functionof temperature as opposed to passing through a clear transitiontemperature. Nonetheless, there can be considerable precipitation ofpolymer upon going to lower temperatures, thus immobilizing orcoagulating the precipitated silver.

The silver products of the processes disclosed herein are useful for thepreparation of a variety of silver inks. The methods for preparation ofvarious inks are specific to the particular applications in which theink will be utilized and are well known to those skilled in the art. Asilver ink is composed of the silver particles and any coating ordispersing materials that result from the processes described herein,dispersed in an ink medium.

As used herein, an “ink medium” for a silver ink is all of thecomponents of the ink other than the silver particles. These componentswill be specific to the printing method (inkjet, screen, gravure,offset, spin, or contact printing) by which the ink will be printed andare well known to those skilled in the art. Examples of ink media aredisclosed herein below.

The ink medium of an aqueous inkjet ink composition typical of theindustry and used as an example for this invention comprises a diol, aglycol ether, and water. It may further comprise a viscoelastic polymercomponent, an inorganic salt, and additional water-miscible solventssuch as 2-pyrrolidinone. More specifically, the ink medium of a typicalinkjet ink compositions preferably comprises the followingconcentrations, expressed as percentage of total ink medium compositionnot including the silver particles:

-   -   (a) 3 to 20 weight percent of at least one diol;    -   (b) 0 to 5 weight percent of at least one glycol ether;    -   (c) 0.5 to 5 weight percent of at least one component selected        from surfactants, buffers, and biocides; and    -   (d) 0.01 to 2 percent of at least one viscoelastic polymer; and    -   (e) the balance water.        While the above-described medium formulation is preferred, any        aqueous-based medium suitable for inkjet ink compositions may be        employed in the formulation of a suitable inkjet ink. The ink        medium is combined with the silver particles formed in the        processes disclosed herein.

Examples of “diols” that may be employed in the preferred inkjet inkmedium include any of, or a mixture of two or more of, such compounds asethanediols (e.g., 1,2-ethanediol); propanediols (e.g., 1,2-propanediol,1,3-propanediol, 2-ethyl2-hydroxymethyl-1,3-propanediol,ethylhydroxypropanediol (EHPD), etc.); butanediols (e.g.,1,3-butanediol, 1,4-butanediol, etc.); pentanediols (e.g.,1,5-pentanediol); and hexanediols (e.g., 1,6-hexanediol, 2,5-hexanediol,etc.). Preferably 1,5-pentanediol and EHPD are employed in the inkmedium.

The “glycol ether” component of the ink medium may comprise any of theglycol ethers and thioglycols ethers, and mixtures thereof, commonlyemployed in inkjet ink compositions. Examples of such compounds includepolyalkylene glycols such as polyethylene glycols (e.g., diethyleneglycol, triethylene glycol, tetraethylene glycol, etc.) polypropyleneglycols (e.g., dipropylene glycol, tripropylene glycol, tetrapropyleneglycol, etc.); polymeric glycols (e.g., PEG 200, PEG 300, PEG 400, PPG400, etc.) and thioglycol. Preferably diethylene glycol is employed inthe ink medium.

Other components that may be employed in the ink medium includesurfactants, buffers, and biocides, each of which are commonly employedadditives in inkjet printing. Surfactants are commonly employed toprevent color to color bleed by increasing the penetration of the inksinto the print medium. Any surfactants suitably employed for thispurpose in inkjet ink compositions may be included in the ink medium.Examples of classes of surfactants that might be employed includeanionic and nonionic surfactants.

“Buffers” can be employed in the ink medium to modulate pH. Preferredare organic-based biological buffers, since inorganic buffers can causeprecipitation of the silver component in the ink composition. Further,the buffer preferably provides a pH ranging from about 6 to 9 for bestresults. Examples of preferred buffers include Trizma Base, which isavailable from, for example, Aldrich Chemical (Milwaukee, Wis.), and4-morpholine ethane sulfonic acid (MES).

A viscoelastic polymer component is included in inkjet ink medium toprevent the formation of aerosol breakoff remnants by increasing theextensional viscosity and surface tension of the ink. “Viscoelasticpolymers” suitably employed in the practice of the invention may be anysuch polymer that increases the extensional viscosity without affectingthe sheer viscosity of the inkjet ink. Examples of classes ofviscoelastic polymers include polyacrylamides, poly(ethylene oxide)s andpoly(vinylpyrrolidones). A mixture of viscoelastic polymers may beemployed. The viscoelastic polymers preferably have a molecular weightfrom about 10,000 to 5,000,000.

Consistent with the requirements of inkjet in media, various other typesof additives may be employed in the ink to optimize the properties ofthe ink composition for specific applications. For example, as is wellknown to those skilled in the art, one or more biocides, fungicides,and/or slimicides (microbial agents) can be used in the ink compositionas is commonly practiced in the art. Examples of microbial agentsinclude, but are not limited to, NUOSEPT® antimicrobial (Nudex, Inc.),UCARCIDE® antimicrobial (Union Carbide), VANCIDE® antimicrobial (RTVanderbilt Co.), and PROXEL® antimicrobial (ICI America). Additionally,sequestering agents such as EDTA may be included to eliminatedeleterious effects of ionic metal impurities.

The process of “spin printing” is described in copending patentapplication US20050089679. An “ink medium” for a spin printing ink inits most simple form consists of a solvent such as water, in which isdissolved between 0.1 and 8 percent by weight of an ultrahigh molecularweight polymer, thereby creating a highly viscoelastic solution. An“ultrahigh molecular weight polymer,” as used herein, is a polymerhaving a molecular weight from 1,000,000 to 20,000,000. The ink mayfurther contain other lower molecular weight polymers (such aspoly(ethyleneglycol) having a molecular weights of from 200 to 50,000)to improve the dispersion, or other solvents to control the rate ofdrying of the ink. The ink medium further contains from 20 to 70 percentby weight of silver particles dispersed therein. The medium may furtherinclude one or more additional components of the inkjet ink describedabove, such as a diol or glycol ether component, surfactants, buffers,and biocides.

The silver compositions described herein are particularly useful forphotoimageable screen printing ink compositions. A photoimageable screenprinting ink comprises the silver particles dispersed in a suitable inkmedium. The main purpose of the ink medium is to serve as a medium fordispersion of the finely divided solids of the composition in such formthat it can readily be applied to a ceramic or other substrate. Thus,the ink medium for the photoimageable screen printing ink is desirablyone in which the solids are dispersible with an adequate degree ofstability. Secondly, the rheological properties of the organic mediumare preferably such that they provide desirable application propertiesto the dispersion. The main components of the ink medium are A)“polymeric binder;” B) “photohardenable monomer;” C. “photoinitiationsystem;” D. “organic solvent;” E) “other additives;” and F) “photospeedenhancer.”

The polymeric binders are important to the compositions ofphotoimageable screen printing inks. They should haveaqueous-developability and give a high resolving power. As used herein,a photocrosslinkable “polymeric binder” is a copolymer of nonacidiccomonomers (i.e.: C1-10 alkyl acrylates or methacrylates, styrenes), andan acidic comonomer (i.e.: ethylenically unsaturated carboxylic acid)wherein 2-20 percent of the carboxylic acid is reacted with a moleculehaving a vinyl group and a second functional unit capable of forming achemical bond by reaction with the carboxylic acid moiety). Examples ofthe vinyl group include, but are not limited to methacrylate andacrylate groups. Examples of the second functional unit include, but arenot limited to epoxides, alcohols and amines. The resultant copolymer,interpolymer or mixture thereof has an acid content of at least 10weight percent of the total polymer weight; a glass transitiontemperature of 50-150° C. and an weight average molecular weight in therange of 2,000-250,000.

As used herein, a “photohardenable monomer” is a monomer added to thesystem for the purpose of undergoing photochemically initiated freeradical polymerization to increase the rigidity of the resulting system.Conventional photohardenable methacrylate monomers can be used in aphotoimageable screen printing ink formulation. Depending on theapplication, it is not always necessary to include a monomer when usingthe photocrosslinkable polymer. Monomer components are present inamounts of 1-20 weight percent, based on the total weight of the dryphotopolymerizable layer. Preferred monomers are disclosed in U.S. Pat.No. 3,380,831 and U.S. Pat. No. 2,927,022.

As used herein, a “photoinitiator is a compound that, in response toactinic radiation, generates free radicals that initiate a free radicalpolymerization within the photoexposed areas. Suitable photoinitiatorsfor photoimageable screen printing inks are those that generate freeradicals upon exposure to actinic light at ambient temperature. Theseinclude the substituted or unsubstituted polynuclear quinones, e.g.,2,2-dimethoxy-2-phenylacetophenone, 9,10-anthraquinone,2-methylanthraquinone, or 2-tert-butylanthraquinone. Otherphotoinitiators that are also useful are described in U.S. Pat. No.2,760,863. Photoreducible dyes and reducing agents disclosed in U.S.Pat. Nos. 2,850,445, 2,875,047, 3,097,096, 3,074,974, 3,097,097 and3,145,104 find utility as do the related compounds described in U.S.Pat. Nos. 3,427,161, 3,479,185, and 3,549,367. Also useful withphotoinitiators and photoinhibitors are sensitizers disclosed in U.S.Pat. No. 4,162,162.

The “organic solvent” component of the ink medium for photoimageablescreen printing inks, which may be a mixture of solvents, is chosen soas to obtain substantially complete solution therein of the polymer andother organic components. The solvent is preferably inert (non-reactive)towards the other components of the composition. For screen printableand photoimageable pastes, the solvent(s) should have sufficiently highvolatility to enable the solvent to be evaporated from the printedimage. Typical examples are 2,2,4-trimethyl-1,3-pentanediolmonoisobutyrate and terpineol both available from Aldrich (St. Louis).

As used herein, the term, “other additives” includes those othersubstances commonly added to screen printing ink systems for the sake ofimproving the performance of the ink. These components, known to thoseskilled in the art may, include dispersants, stabilizers, release,agents, dispersing agents, stripping agents, and antifoaming agents.Examples of suitable materials are disclosed in U.S. Pat. No. 5,049,480.

Frequently the ink medium for photoimageable screen printing inkscontains one or more plasticizers if additional film softness is needed.Such plasticizers help to assure good lamination to substrates andenhance the developability of unexposed areas of the composition.

A “photospeed enhancer” is a material added to the ink medium forphotoimageable screen printing inks to increase the effectiveness ofradicals generated for polymerization. Examples of photospeed enhancersinclude stearic acid, palmitic acid, salts of stearate, and salts ofpalmitate, wherein the counter-ion can be ammonium, sodium and potassiumor mixtures thereof.

EXAMPLES

The following examples illustrate the embodiments of the invention thatare presently best known. However, it is to be understood that thefollowing arrangements are only illustrative of the application of theprinciples of the present invention. Numerous modifications andalternative arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the present invention andthe appended claims are intended to cover such modifications andarrangements. Thus, the present invention has been described above withparticularity and the following Examples provide further detail inconnection with what are presently deemed to be the most practical andpreferred embodiments of the invention. Nonetheless, it will be apparentto those skilled in the art that numerous modifications, including, butnot limited to, variations in size, materials, shape, form, function andmanner of operation, assembly and use may be made without departing fromthe principles and concepts set forth herein.

The purity of all components used in the following examples is thatfound in common commercial practice.

Some polymers were from Scientific Polymer Products, which is located inOntario, N.Y. Most other materials were from Aldrich Chemical located inSt. Louis, Mo. The roller mill employed was by U.S. Stoneware Corp.,Palestine Ohio) and was set on its lowest speed.

Example 1 Preparation of Silver with poly(N-isopropyl acrylamide)

An aqueous solution of poly(isopropylacrylamide) (1.25 g, ScientificPolymer Products, MW=300,000) in deionized water (100 mL) was preparedon a roller mill. Silver nitrate (Aldrich) was introduced to the polymersolution by dissolving 16.99 g (0.10 mol, MW=169.88) of silver nitrate.This mixture was stirred until all had gone into solution. To this wasadded methylamine (20 mL, Aldrich, 40 percent aqueous solution, 17.94 g,0.231 mol) with rapid stirring. There was an immediate precipitation ofa tan material that rapidly went back into solution giving a relativelyclear, beige solution.

A solution of the formaldehyde (20 mL, Aldrich 37 percent aqueoussolution containing methanol, 18.34 g, 0.224 mol) was diluted to 100 mLwith water. The solution was quickly added to the silver methylaminesolution. There was an immediate reaction precipitating silver and thenthe reaction mass separated from the solution as a viscous, low-densitymass. The precipitation was probably the result of the reaction exothermtaking to polymer solution temperature above the point where the polymerprecipitated. The supernatent was clear and colored slightly tan. It wasdecanted from the reaction mixture.

The reaction mass was re-suspended in 100 mL of cool water. Withstirring it became a mobile slurry with most if not all of the polymergoing into solution. The solution was then heated in a water bath withstirring until the polymer once again precipitated, taking the silverout of suspension. The supernatent was carefully decanted leaving beigemass. This procedure was repeated two more times, washing the productwith deionized water each time.

Example 2 Preparation with poly(N-isopropyl acrylamide)

To an aqueous solution of 20.0 mL polyacrylamide (50 percent watersolution of molecular weigh 1500) in deionized water (100 mL) in a glassbottle was added poly(isopropylacrylamide) (1.25 g, Scientific PolymerProducts, MW=300,000) and the resulting suspension was placed on aroller mill for 24 hours. The polymer did not go into solution andanother 100 mL of water was added to the bottle. Further roller-milling(24 hr) still did not give a solution. Silver nitrate was introduced tothe polymer solution by dissolving 16.99 g (0.10 mol, MW=169.88) ofsilver nitrate. The suspension whitened and thepoly(isopropylacrylamide) still did not go into solution. This mixturewas added slowly to a solution of methylamine (20 mL, Aldrich 40 percentaqueous solution, 17.94 g, 0.231 mol) with rapid stirring. There was animmediate precipitation of a tan material that rapidly went back intosolution giving a relatively beige suspension.

A solution of the formaldehyde (20 mL, Aldrich 37 percent aqueoussolution containing methanol, 18.34 g, 0.224 mol) was diluted with water(100 mL) in a plastic beaker. The silver methylamine solution wasquickly added to the formaldehyde solution. There was a slow reactionthat turned reddish and dark. The suspension was stirred for an hour.

Heating the suspension to 50° C. caused the reaction mass to separatefrom the solution as a viscous, low-density material. It was easilyisolated by decantation. The supernatent was dark red-black.Centrifugation of the supernatent yielded a very small quantity ofsilver and the supernatent from the centrifugation remained the darkred-black.

The reaction mass was re-suspended in 100 mL of cool water. Withstirring it became a mobile slurry with most if not all of the polymergoing into solution. The solution was then heated in a water bath withstirring until the polymer once again precipitated, taking the silverout of suspension. The supernatent was carefully decanted leaving darkmass. This procedure was repeated one more time, washing the productwith deionized. This was substantially all of the silver from thereaction. Electron microscopy of the particles showed that there wereprimary particles in the range of 100 nm and they appeared to besingle-crystalline.

Example 3 Control with no poly(acrylamide)

As a Control for example 2, the experiment was repeated without thesupporting polymer, poly(acrylamide) to control the particle size.

To deionized water (100 mL) in a glass bottle was addedpoly(N-isopropylacrylamide) (1.25 g, Scientific Polymer Products,MW=300,000) and the resulting suspension was placed on a roller mill for24 hours. The polymer did not go completely into solution and another100 mL of water was added to the bottle. Further roller-milling (24 hr)gave a solution. Silver nitrate was introduced to the polymer solutionby dissolving 16.99 g (0.10 mol, MW=169.88) of silver nitrate. There wasno color change. This mixture was added slowly to a solution ofmethylamine (20 mL, Aldrich 40 percent aqueous solution, 17.94 g, 0.231mol) with rapid stirring. There was an immediate precipitation of a tanmaterial that rapidly went back into solution giving a relatively clear,beige solution.

A solution of the formaldehyde (20 mL, Aldrich 37 percent aqueoussolution containing methanol, 18.34 g, 0.224 mol) was diluted with water(100 mL). The silver methylamine solution was quickly added to theformaldehyde solution in a plastic beaker. There was a rapid reactionthat evolved gas and rapidly turned black. A porous black polymer massfloated to the top of the reaction. The suspension was stirred for anhour and the black mass remained on the top. Intense stirring was ableto break it up slightly.

Heating the suspension to 50° C. caused the reaction mass to separatecleanly from the solution as a viscous, low-density material thatfloated on top of the reaction solution. It was easily isolated bydecantation. The supernatent was relatively dark tan. Centrifugation ofthe supernatent yielded a very small quantity of silver and thesupernatent from the centrifugation remained the tan.

The reaction mass was re-suspended in 100 mL of cool water. Withstirring it became a mobile slurry with most if not all of the polymergoing into solution. The solution was then heated in a water bath withstirring until the polymer once again precipitated, taking the silverout of suspension. This time the mass was more dense than the solutionand was at the bottom of the container. The supernatent was carefullydecanted leaving dark mass. This procedure was repeated one more time,washing the product with deionized. This was substantially all of thesilver from the reaction. Electron microscopy of the particles showedthat they were agglomerated spheres in the 500 nm range though thosespheres could contain smaller particles.

Example 4 Control with no poly(isopropylacrylamide)

As a control for Example 2, the process was carried out without thethermomorphic poly(N-isopropylacrylamide).

An aqueous solution of 20.0 mL polyacrylamide (50 percent water solutionof molecular weight 1500) was mixed with deionized water (100 mL) in aglass bottle and the resulting solution was placed on a roller mill for24 hours. Another 100 mL of water was added to the bottle androller-milling was continued another 24 hr. Silver nitrate wasintroduced to the polymer solution by dissolving 16.99 g (0.10 mol,MW=169.88) of silver nitrate. The suspension whitened. This mixture wasadded slowly to a solution of methylamine (20 mL, Aldrich 40 percentaqueous solution, 17.94 g, 0.231 mol) with rapid stirring. There was animmediate precipitation of a tan material that rapidly went back intosolution giving a beige suspension.

A solution of the formaldehyde (20 mL, Aldrich 37 percent aqueoussolution containing methanol, 18.34 g, 0.224 mol) was diluted with water(100 mL). The silver methylamine solution was quickly added to theformaldehyde solution. There was a slow reaction that turned reddish anddark. The reaction may have been slightly faster than that of 101. Thesuspension was stirred for an hour.

Heating the suspension to 50° C. resulted in no change. The liquidremained dark red-black and it was not possible to carry out anyseparation. Isolation could not be accomplished by decantation orfiltration. Centrifugation of the suspension yielded the expectedquantity of silver and the supernatent from the centrifugation remainedthe dark red-black.

The reaction mass was re-suspended in 100 mL of water. With stirring itbecame a somewhat mobile slurry. The suspension was then heated in awater bath with stirring to no effect. The suspension was once morecentrifuged to isolate the solids. Electron microscopy of the particlesshowed that the primary particles were in the range of 100 nm and theyappeared to be single-crystalline, but they were massed together by theisolation process and could not be readily redispersed.

Example 5 Modification of Order of Addition

As an improvement for Example 2 the order of addition of the reagentswas changed, yielding a more scalable process.

A suspension of poly(isopropylacrylamide) (1.25 g, Scientific PolymerProducts, MW=300,000) in water (200 mL) was prepared in a glass bottleand the resulting suspension was placed on a roller mill for 48 hours. Aclear solution was obtained. To that solution was added an aqueoussolution of 20.0 mL polyacrylamide (50 percent water solution ofmolecular weigh 1500). The solution remained relatively clear thoughthere was a slight change. Thus the order of addition of the twopolymers makes a difference when making a solution. Silver nitrate wasintroduced to the polymer solution by dissolving 16.99 g (0.10 mol,MW=169.88) of silver nitrate. The suspension whitened like thew otherreactions in which polyacrylamide was present. This mixture was addedslowly to a solution of methylamine (20 mL, Aldrich 40 percent aqueoussolution, 17.94 g, 0.231 mol) with rapid stirring. There was animmediate precipitation of a tan material that rapidly went back intosolution giving a relatively beige suspension.

A solution of the formaldehyde (20 mL, Aldrich 37 percent aqueoussolution containing methanol, 18.34 g, 0.224 mol) was diluted with water(100 mL) in a plastic beaker. The silver methylamine solution wasquickly added to the formaldehyde solution. There was a slow reactionthat turned reddish and dark. The suspension was stirred for an hour.

Heating the suspension to 50° C. caused the reaction mass to separatefrom the solution as a viscous, low-density material. It was easilyisolated by decantation. The supernatent was dark red-black.Centrifugation of the supernatent yielded a very small quantity ofsilver (E105129-104A) and the supernatent from the centrifugationremained the dark red-black.

The reaction mass was re-suspended in 100 mL of cool water. Withstirring it became a mobile slurry with most if not all of the polymergoing into solution. The solution was then heated in a water bath withstirring until the polymer once again precipitated, taking the silverout of suspension. The supernatent was carefully decanted leaving darkmass. This procedure was repeated one more time, washing the productwith deionized water. This was substantially all of the silver from thereaction.

This overall reaction was very much like 101 other than the fact thatthe poly(isopropylacrylamide) in 101 was very non-homogeneous. Electronmicroscopy of the particles showed that there were primary particles inthe range of 100 nm and they appeared to be single-crystalline but theywere totally surrounded by polymer.

Example 6 Formulation of a Screen Printing Ink

Preparation of the media: The solvent Texanol® (Union Carbide)(11 g) andacrylic polymers PVPNA S-630 (a copolymer of 60 percent vinylpyrrolidoneand 40 percent vinyl acetate. K-value=30-50 (ISP Technologies, Inc.,Wayne, N.J.))(0.3 g) and Carboset XPD1234, a coplymer of 80 percentmethyl methacrylate and 20 percent methacrylic acid, average weightmolecular weight Mw=6,000, acid number=120 (B. F. Goodrich)) (7.2 g)were mixed and heated with stirring to 80° C. Heating and stirring wascontinued until the acrylic polymer had dissolved. The solution was thenallowed to cool.

A glass frit available from DuPont (Wilmington, Del.) had the componentweight percent of the following: SiO₂ (9.1), Al₂O₃ (1.4) PbO (77.0),B₂O₃ (12.5).

A silver was prepared as in Example 2. After washing was completed, thedamp coagulant was mixed with 0.5 weight percent stearic acid (basedupon silver) and was mixed for 15 minutes. The slurry was used as is inthe next step.

The silver (6.8 g based upon silver) and frit (0.2 g) were then added to2 g of the organic medium mixture. The total composition was then mixedthoroughly. The mixture was then further wetted with an additional 0.3 gof Taxanol® and mulled on a Hoover Automatic Muller, Model M5.

The resulting ink was suitable for screen printing. Care should be takento avoid dirt contamination in the process of preparing pastecompositions and in preparing parts, since such contamination can leadto defects. The paste can be applied to a glass substrate byscreen-printing using 325 mesh screens. Addition of a photoinitiatingpackage and a photopolymerizing polymer system would have made the inkphotodevelopable.

Example 7 Formulation of an Inkjet Ink

A silver sample was prepared as in Example 2. The sample was washedthree times by redissolving in fresh deionized water containing 0.5percent stearic acid and then heating to a temperature above thethermomorphic transition to reprecipitate the solid coagulant. Afterwashing was completed, the coagulant mixture (4.0 g based upon silver)was mixed with de-ionized water (4.0 g) and 0.5 weight percent stearicacid (based upon silver) was added to the slurry as a coating material.Mixing was continued for 15 minutes.

The stearate-treated coagulant was combined with Dowanol®-DB (DowChemical, Midland Mich.) (2 g), cetyltrimethylammonium bromide (Aldrich)(0.2 g), Igepal CA-210 (Aldrich) (0.3 g), and Solsperse 44000 (Avecia,Manchester, UK). Water was added until the total weight was 10 g. Thesample was then ultrasonically dispersed for 30 min (BransonUltrasonics, Danbury Conn., Digital Sonifier with a CE converter set atpower level 4) with an ice/water bath for cooling.

The resulting ink was suitable for inkjet printing.

1. A process for preparing and isolating silver particles or colloidscomprising: a) providing, at a first temperature, a combined mixturecomprising 1) a first aqueous solution of a silver(I) salt and an amineand 2) a second aqueous solution comprising a reducing agent; saidcombined mixture further comprising a thermomorphic polymer having atransition temperature, said thermomorphic polymer being in a homogenousphase at said first temperature; and b) changing the temperature of thecombined solution from the first temperature to a second temperature atwhich the thermomorphic polymer is in a heterogeneous phase, such thatthe thermomorphic polymer and silver separate from the combined solutionto form an agglomerate; and c) isolating the agglomerate from thereaction medium.
 2. The process of claim 1 wherein said silver(I) salt,said amine and said thermomorphic polymer are provided in said firstsolution, and said reducing agent is provided in said second solution,and said combined mixture is formed by contacting the first solution andthe second solution.
 3. The process of claim 1 wherein said silver(I)salt and said amine are provided in said first solution, and saidthermomorphic polymer and said reducing agent are provided in saidsecond solution, and said combined mixture is formed by contacting thefirst solution and the second solution.
 4. The process of claim 1wherein said silver(I) salt and said amine are provided in a firstsolution, and said reducing agent is provided in a second solution; saidfirst solution and said second solution are combined to form anadmixture; and said admixture is contacted with said thermomorphicpolymer to form said combined solution prior to changing the temperatureof the combined solution from the first temperature to a secondtemperature at which the thermomorphic polymer is in a heterogeneousphase.
 5. The process of claim 1 wherein the first or second aqueoussolution also comprises one or materials selected from supportingpolymers and surfactants.
 6. The process of claim 1 wherein theisolation is by decantation or filtration.
 7. The process of claim 1,further comprising washing the agglomerate by: a) redissolving theagglomerate in a fresh aqueous washing medium at a temperature that ison the homogeneous phase side of the transition temperature of saidthermomorphic polymer, to form a solution; b) changing the temperatureof the solution through the transition temperature of the thermomorphicpolymer to a temperature at which the thermomorphic polymer solution isin its heterogeneous phase causing the silver particles andthermomorphic polymer to reform an agglomerate; and c) re-isolating theagglomerate from the aqueous washing medium.
 8. The process of claim 7wherein the washing medium comprises a surfactant.
 9. The process ofclaim 1 wherein the silver salt is silver nitrate, silvertrifluoroacetate, silver oxide or silver acetate.
 10. The process ofclaim 1 wherein the reducing agent is one or more compounds selectedfrom: formaldehyde, hydroquinone, ascorbic acid, reducing sugars,hydrazines, hydroxylamines, dimethylformamide, ethylene glycol, andaldehydes.
 11. The process of claim 1 wherein the amine is selected frommethylamine, ethylamine, ethanolamine, ethylenediamine, dimethylamine,and diethanolamine.
 12. The process of claim 1 wherein the thermomorphicpolymer is poly(isopropylacrylamide).
 13. The process of claim 1 whereinthe silver salt is silver nitrate, the reducing agent is formaldehyde,the amine is methylamine, and the thermomorphic polymer ispoly(isopropylacrylamide).
 14. The process of claim 1 wherein thesupporting polymer is polyacrylamide or poly(isopropylacrylamide) or acombination thereof.
 15. Silver particles prepared and isolated by aprocess comprising: a) providing, at a first temperature, a combinedmixture comprising 1) an aqueous solution of a silver(I) salt and anamine and 2) a second aqueous solution comprising a reducing agent; saidcombined mixture further comprising a thermomorphic polymer having atransition temperature, said thermomorphic polymer being in a homogenousphase at said first temperature; and b) changing the temperature of thecombined solution from the first temperature to a second temperature atwhich the thermomorphic polymer is in a heterogeneous phase, such thatthe thermomorphic polymer and silver separate from the combined solutionto form an agglomerate; and c) isolating the agglomerate from thereaction medium.
 16. The silver particles of claim 15 wherein saidsilver(I) salt, said amine and said thermomorphic polymer are providedin said first solution, and said reducing agent is provided in saidsecond solution, and said combined mixture is formed by contacting thefirst solution and the second solution.
 17. The silver particles ofclaim 15 wherein said silver(I) salt and said amine are provided in afirst solution, and said thermomorphic polymer and said reducing agentare provided in a second solution, and said combined mixture is formedby contacting the first solution and the second solution.
 18. The silverparticles of claim 15 wherein said silver(I) salt and said amine areprovided in a first solution, and said reducing agent is provided in asecond solution; said first solution and said second solution arecombined to form an admixture; and said admixture is contacted with asaid thermomorphic polymer to form said mixture prior to changing thetemperature of the combined solution from the first temperature to asecond temperature at which the thermomorphic polymer is in aheterogeneous phase.
 19. The silver particles of claim 15 wherein theaqueous solution also comprises one or more materials selected fromsupporting polymers and surfactants.
 20. The silver particles of claim15 wherein the process includes isolating the agglomerate by decantationor filtration.
 21. The silver particles of claim 15 wherein the processfurther comprises washing the agglomerate by: a) redissolving theagglomerate in a fresh aqueous washing medium at a temperature that ison the homogeneous phase side of the transition temperature of saidthermomorphic polymer, to form a solution; b) changing the temperatureof the solution through the transition temperature of the thermomorphicpolymer to a temperature at which the thermomorphic polymer solution isin its heterogeneous phase causing the silver particles andthermomorphic polymer to reform an agglomerate; and c) re-isolating theagglomerate from the aqueous washing medium.
 22. The silver particles ofclaim 21 wherein the washing medium comprises a surfactant.
 23. Thesilver particles of claim 15 wherein the silver salt is silver nitrate,silver trifluoroacetate, silver oxide or silver acetate.
 24. The silverparticles of claim 15 wherein the reducing agent comprises one or morecompounds selected from formaldehyde, hydroquinone, ascorbic acid,reducing sugars, hydrazines, hydroxylamines, dimethylformamide, ethyleneglycol, and aldehydes.
 25. The silver particles of claim 15 wherein theamine utilized in the process is selected from methylamine, ethylamine,ethanolamine, ethylenediamine, dimethylamine, and diethanolamine. 26.The silver particles of claim 15 wherein the thermomorphic polymer ispoly(isopropylacrylamide).
 27. The silver particles of claim 15 whereinthe silver salt of the process is silver nitrate, the reducing agent isformaldehyde, the amine is methylamine, and the thermomorphic polymer ispoly(isopropylacrylamide).
 28. The silver particles of claim 15 whereinthe supporting polymer is polyacrylamide or poly(isopropylacrylamide) ora combination thereof.
 29. A printing ink comprising silver particles ofclaim
 15. 30. A process of preparing a silver ink comprising: a)providing, at a first temperature, a combined mixture comprising 1) anaqueous solution of a silver(I) salt and an amine and 2) a secondaqueous solution comprising a reducing agent; said combined mixturefurther comprising a thermomorphic polymer having a transitiontemperature, said thermomorphic polymer being in a homogenous phase atsaid first temperature; and b) changing the temperature of the combinedsolution from the first temperature to a second temperature at which thethermomorphic polymer is in a heterogeneous phase, such that thethermomorphic polymer and silver separate from the combined solution toform an agglomerate; and c) isolating the agglomerate: and d) dispersingsaid agglomerate in an ink medium.
 31. The process of claim 30 whereinsaid silver(I) salt, said amine and said thermomorphic polymer areprovided in said first solution, and said reducing agent is provided insaid second solution, and said combined mixture is formed by contactingthe first solution and the second solution.
 32. The process of claim 30wherein said silver(I) salt and said amine are provided in a firstsolution, and said thermomorphic polymer and said reducing agent areprovided in a second solution, and said combined mixture is formed bycontacting the first solution and the second solution.
 33. The processof claim 30 wherein said silver(I) salt and said amine are provided in afirst solution, and said reducing agent is provided in a secondsolution; said first solution and said second solution are combined toform an admixture; and said admixture is contacted with a saidthermomorphic polymer to form said mixture prior to changing thetemperature of the combined solution from the first temperature to asecond temperature at which the thermomorphic polymer is in aheterogeneous phase.
 34. The process of claim 30 wherein the first orsecond aqueous solution also comprises one or more materials selectedfrom supporting polymers and surfactants.
 35. The process of claim 30wherein the isolation is by decantation or filtration.
 36. The processof claim 30, further comprising washing the polymer/silver agglomerateprior to the step of dispersing the product in an ink medium by: a)redissolving the agglomerate in a fresh aqueous washing medium at atemperature that is on the homogeneous phase side of the transitiontemperature of said thermomorphic polymer to form a solution; b)changing the temperature of the solution through the transitiontemperature of the thermomorphic polymer to a temperature at which thethermomorphic polymer solution is in its heterogeneous phase causing thesilver particles and thermomorphic polymer to reform an agglomerate; andc) re-isolating the agglomerate from the aqueous washing medium.
 37. Theprocess of claim 36 wherein the washing medium comprises a surfactant.38. The process of claim 30 wherein the silver salt is silver nitrate,silver trifluoroacetate, silver oxide or silver acetate.
 39. The processof claim 30 wherein the reducing agent is one or more compounds selectedfrom formaldehyde, hydroquinone, ascorbic acid, reducing sugars,hydrazines, hydroxylamines, dimethylformamide, ethylene glycol, andaldehydes.
 40. The process of claim 30 wherein the amine is selectedfrom methylamine, ethylamine, ethanolamine, ethylenediamine,dimethylamine, and diethanolamine.
 41. The process of claim 30 whereinthe thermomorphic polymer is poly(isopropylacrylamide).
 42. The processof claim 30 wherein the silver salt is silver nitrate, the reducingagent is formaldehyde, the amine is methylamine, and the thermomorphicpolymer is poly(isopropylacrylamide).
 43. The process of claim 30wherein the supporting polymer is polyacrylamide orpoly(isopropylacrylamide) or a combination thereof.
 44. The process ofclaim 30 wherein the ink is for inkjet, screen, gravure, offset, spin,or contact printing.
 45. A silver ink composition prepared by a processcomprising: a) providing, at a first temperature, a combined mixturecomprising 1) an aqueous solution of a silver(I) salt and an amine and2) a second aqueous solution comprising a reducing agent; said combinedmixture further comprising a thermomorphic polymer having a transitiontemperature, said thermomorphic polymer being in a homogenous phase atsaid first temperature; and b) changing the temperature of the combinedsolution from the first temperature to a second temperature at which thethermomorphic polymer is in a heterogeneous phase, such that thethermomorphic polymer and silver separate from the combined solution toform an agglomerate; c) isolating the agglomerate from the reactionmedium; and d) dispersing said agglomerate in an ink medium.
 46. Thesilver ink composition of claim 45 wherein said silver(I) salt, saidamine and said thermomorphic polymer are provided in said firstsolution, and said reducing agent is provided in said second solution,and said combined mixture is formed by contacting the first solution andthe second solution.
 47. The silver ink composition of claim 45 whereinsaid silver(I) salt and said amine are provided in a first solution, andsaid thermomorphic polymer and said reducing agent are provided in asecond solution, and said combined mixture is formed by contacting thefirst solution and the second solution.
 48. The silver ink compositionof claim 45 wherein said silver(I) salt and said amine are provided in afirst solution, and said reducing agent is provided in a secondsolution; said first solution and said second solution are combined toform an admixture; and said admixture is contacted with a saidthermomorphic polymer to form said mixture prior to changing thetemperature of the combined solution from the first temperature to asecond temperature at which the thermomorphic polymer is in aheterogeneous phase, such that the thermomorphic polymer and silverseparate from the combined solution to form an agglomerate; andisolating the agglomerate from the reaction medium.
 49. The silver inkcomposition of claim 45 wherein the first or second aqueous solution ofthe process also comprises one or materials selected from supportingpolymers and surfactants.
 50. The silver ink composition of claim 45wherein the process includes isolation by decantation or filtration. 51.The silver ink composition of claim 45 wherein the process furthercomprises washing the agglomerate prior to dispersing the product in anink medium by: a) redissolving the agglomerate in a fresh aqueouswashing medium at a temperature that is on the homogeneous phase side ofthe transition temperature of said thermomorphic polymer; b) changingthe temperature of the solution through the transition temperature ofthe thermomorphic polymer to a temperature at which the thermomorphicpolymer solution is in its heterogeneous phase causing the silverparticles and thermomorphic polymer to reform an agglomerate; and c)re-isolating the agglomerate from the aqueous washing medium.
 52. Theink composition of claim 51 wherein the washing medium comprises asurfactant.
 53. The ink composition of claim 45 wherein the silver saltis silver nitrate, silver trifluoroacetate, silver oxide or silveracetate.
 54. The ink composition of claim 45 wherein the reducing agentis one or more compounds selected from formaldehyde, hydroquinone,ascorbic acid, reducing sugars, hydrazines, hydroxylamines,dimethylformamide, ethylene glycol, and aldehydes.
 55. The inkcomposition of claim 45 wherein the amine is selected from methylamine,ethylamine, ethanolamine, ethylenediamine, dimethylamine, anddiethanolamine.
 56. The ink composition of claim 45 wherein thethermomorphic polymer is poly(isopropylacrylamide).
 57. The inkcomposition of claim 45 wherein the silver salt is silver nitrate, thereducing agent is formaldehyde, the amine is methylamine, and thethermomorphic polymer is poly(isopropylacrylamide).
 58. The inkcomposition of claim 45 wherein the supporting polymer is polyacrylamideor poly(isopropylacrylamide) or a combination thereof.
 59. A processcomprising printing an image using the ink composition of claim 45,wherein said process comprises inkjet, screen, gravure, offset, spin, orcontact printing.
 60. The ink composition of claim 45 wherein the mediumcomprises (a) 3 to 20 weight percent of at least one diol; (b) 0 to 5weight percent of at least one glycol ether; (c) 0.5 to 5 weight percentof at least one component selected from surfactants, buffers, andbiocides; (d) 0.01 to 2 weight percent of at least one viscoelasticpolymer; and (e) the balance water.
 61. A process of claim 59 whereinsaid printing comprises inkjet printing.
 62. The ink composition ofclaim 45 wherein the medium comprises (a) 0.1 to 8 weight percent of atleast one ultrahigh molecular weight polymer; (b) 0 to 5 weight percentof at least one glycol ether; (c) 0 to 5 weight percent of one or morepoly(ethyleneglycol)s of molecular weights from 200 to 50,000; (d) 0.5to 5 weight percent of at least one component selected from surfactants,buffers, and biocides; (e) the balance water.
 63. A process comprisingspin printing using the ink composition of claim
 62. 64. The inkcomposition of claim 45 wherein the medium comprises (a) 30 to 50 weightpercent of a polymeric binder; (b) 0 to 5 weight percent of at least onephotohardenable monomer; (c) 1 to 5 weight percent of photoinitiationsystem; (d) 0.5 to 5 weight percent of at least one component selectedfrom surfactants, buffers, and biocides; (e) 0 to 1 weight percent of aphotospeed enhancer; and (f) the balance an organic solvent.
 65. Aprocess comprising screen printing using the ink composition of claim64.